How to install audiocraft on NixOS

how do I install an equivalent to python309Packages.av - this is related to one of the tens of random errors encountered when building this (but not especially this) package on NixOS
I looked through git commit history for about 30 seconds and decided I only knew how to run nix shell github:cachix/nixpkgs-python#'"3.9"' or downgrade packages from within configuration.nix anyway so I’m posting this here.
How do I install av for audiocraft? It seems python312/313 versions don’t support python3.9
If this ends up being more complicated than this is could at least help me figure out how to built blender from source — I keep running into similar issues, usually over a quarter of them stemming from pkg-config which I can fix about fifty of via adding to path variables before on pops up that ignores path variables leaving it impossible to build the package.

How to build audiocraft from source on NixOS
and/or how to build blender from source on NixOS